Modern pulsed, high repetition rate, gas discharge lasers such as ArF XeF and Krf excimer lasers and molecular fluorine lasers generally employ a pair of spaced apart, elongated (e.g. 30 cm long) main discharge electrodes to initiate lasing in a gaseous material. For example, each pulse may be produced by applying a very high, voltage potential across the electrodes with a power supply which causes a discharge between the electrodes having a duration of about 30 nanoseconds. A typical discharge may deposit about 2.5 J of energy into a gain region that is about 20 mm high, 3 mm wide and 500 mm long.
Each discharge alters the physical condition of the gas in the discharge region rendering the gas unsuitable for use in the next pulse. For this reason, a circulation system, which may include a high-speed cross-flow type fan, is typically provided to quickly exhaust “spent” gas from the discharge region immediately after a pulse and present a fresh portion of gas to the electrodes for the next pulse. Thus, at a pulse repetition rate in the range of 1000 Hz or greater, relatively high gas velocities are required to completely clear spent gas and debris prior to the next pulse.
In the absence of suitable precautions, the relatively high gas velocities that are required to exhaust all of the “spent” discharge gas are capable of creating turbulent flow. In particular, turbulence can develop near the discharge electrodes and cause undesirable arcing between the electrodes. This arcing, in turn, may result in poor laser performance, including, but not limited to, lowered pulse energy and lowered pulse-to-pulse energy stability. Heretofore, discharge chambers have been disclosed which include various permanently installed, non-adjustable, baffles, vanes and/or fairings to improve the aerodynamic geometry of the chamber and to reduce turbulence in the flow of laser gas. These features have also included non-adjustable fairings to minimize turbulent flow in and around the discharge electrodes, for example see U.S. Pat. No. 6,914,919, issued on Jul. 5, 2005 and titled, “Six To Ten KHZ, Or Greater Gas Discharge Laser System”.
For the above-described arrangement (i.e. high voltage, high repetition rate gas discharge laser), erosion and/or other wear mechanisms that are operable during electrode discharge may cause one or both of the electrodes to lose mass and physically shorten over the life of a laser. Indeed, currently available gas discharge lasers may have chambers having a useful life of 12 billion pulses or more. The significant shortening of the electrode (which may occur after only a half billion pulses or less) may create a geometrical mis-match between the electrode and any fairing or other surface feature that is provided to minimize turbulence in the flow of laser gas passing by the electrode. Although a small mis-match may be tolerable, larger mis-matches between the electrode and fairing can result in undesirable turbulent flow, arcing, and a corresponding reduction in laser performance.
With the above considerations in mind, Applicants disclose a laser and methods for operating a laser that adjust laser gas flow over the life of the laser to accommodate electrode shortening due to erosion.